The present invention relates to a semiconductor device including a MOS field effect transistor having a gate electrode of a stacked structure including a polycrystalline silicon (polysilicon) layer and a metal layer and a method of manufacturing the same.
In recent years, a MOS field effect transistor (hereinafter referred to as MOS-FET) is being miniaturized, and the operating speed of the MOS-FET is being promoted. With increase in the operating speed, a problem of signal delay, i.e., gate delay, is generated. The gate delay depends on the product between the capacitance and the resistance of the gate wiring. In order to suppress the gate delay, used is a gate electrode of a stacked structure consisting of a polysilicon layer and a metal layer, e.g., a stacked structure consisting of a polysilicon layer and a tungsten (W) layer.
FIG. 1 is a cross sectional view showing the construction of a conventional semiconductor device having a gate electrode of a stacked structure consisting of a polysilicon layer and a tungsten layer. As shown in the drawing, a gate insulating film 101 is formed on a semiconductor substrate 100, and a gate electrode of a stacked structure consisting of a polysilicon layer 102 and a tungsten layer 103 is formed on the gate insulating film 101.
In manufacturing a semiconductor device provided with such a gate electrode, the tungsten layer 103 tends to be oxidized under an oxidizing atmosphere or tends to be dissolved in a process solution consisting of sulfuric acid and hydrogen peroxide solution. To overcome these difficulties, the gate electrode is covered with a cap film 104 and a gate side wall film 105. Each of these cap film 104 and gate side wall film 105 consists of a silicon nitride film. When it comes to the manufacturing process of the device shown in FIG. 1, a resist removing step is performed after the etching step by lithography in preparation for a wiring step with, for example, aluminum. In this resist removing step, used is a mixed solution consisting of sulfuric acid and hydrogen peroxide solution.
It should also be noted that it is important to decrease the parasitic resistance in order to achieve further miniaturization of the semiconductor device for increasing the degree of integration and to allow the semiconductor device to be operated at a high speed. In view of these requirements, used is a salicide technology that is effective for decreasing the diffusion layer resistance and the contact resistance. In the salicide technology, a metal such as titanium or cobalt is deposited on a diffusion layer, followed by applying a heat treatment so as to bring about reaction between silicon in the diffusion layer and the deposited metal, thereby forming a silicide layer in the diffusion layer.
The salicide technology includes a selective etching step for selectively removing the unreacted metal, with the silicide formed by the heat treatment left unremoved. A mixed solution consisting of sulfuric acid and hydrogen peroxide solution is used in this selective etching step.
As described above, the gate electrode of a stacked structure consisting of a polysilicon layer and a tungsten layer is treated in a subsequent step with a chemical solution containing hydrogen peroxide solution. What should be noted is that tungsten is dissolved in the particular chemical solution, making it necessary to cover the tungsten layer with an insulating film.
Tungsten is poor in its resistance to oxidation. Therefore, it is desirable for the insulating film to be formed of a material that can be deposited under a reducing atmosphere and that is capable of inhibiting intrusion of an oxidizing agent in the subsequent heating step. In general, the insulating film is formed of silicon nitride.
However, defects such as pin holes tend to be formed by stress in the silicon nitride film. Naturally, defects such as pin holes are formed in many cases in the silicon nitride film covering the gate electrode, making it difficult to prevent a mixed solution consisting of sulfuric acid and hydrogen peroxide solution from permeating through the pin holes so as to dissolve tungsten in the selective etching step with the mixed solution in the subsequent step of forming a silicide layer in the source and drain regions (diffusion layers). It should also be noted that the removing solution for removing the resist film used for the patterning intrudes through the pin holes made in the silicon nitride film used as a gate protective film (cap film and gate side wall film) so as to dissolve tungsten and, thus, to bring about breakage of the gate electrode.
Further, when a silicon nitride film acting as a cap film is deposited on the tungsten layer, tungsten is oxidized by the oxidizing agent within the atmosphere so as to bring about a morphological deterioration of the surface.
Still further, when a silicon nitride film is deposited to form a gate side wall film on the side surface of the gate electrode, an oxidizing agent within the atmosphere tends to intrude through the defects such as pin holes of the silicon nitride film so as to oxidize the tungsten layer included in the gate electrode.